Rikkyo University

Triplet-triplet annihilation photon upconversion (TTA-UC) is a method of converting long-wavelength light into short-wavelength light by combining a sensitizer with a component called an emitter. This technique has attracted a great deal of attention for the conversion of light energy. If it were possible to efficiently convert near-infrared light, which is abundant in sunlight, to ultraviolet or visible light, it could dramatically improve the efficiency of devices and materials such as solar cells and hydrogen generation photocatalysts.
The mechanism of TTA-UC in solution is as follows (Figure 2). First, the base (S₀) state sensitizer absorbs long-wavelength light to create an excited singlet (S₁) state, which then goes through intersystem crossing (ISC) to reach the excited triplet (T₁) state. Triplet energy transfer (TET) occurs between the T₁ state sensitizer and the S₀ state emitter, producing a T₁ state in the emitter (triplet sensitization). Because the life of the emitter T₁ state is relatively long, they meet to cause triplet-triplet annihilation (TTA), one returning to the high-energy S₁ state and the other to the S₀ state.

Their research focused on metal clusters protected by organic ligands, which are novel sensitizers that are both stable and low-toxicity. With ligand-protected metal clusters, it is possible to precisely synthesize the number and composition of constituent atoms down to the atomic level. These clusters come in many varieties capable of absorbing light up to the near-infrared region. In their research, silver (Ag) clusters [Ag₂₅(SR)₁₈] composed of silver atoms protected by a thiolate ligand (SR) and silver-platinum alloy clusters [PtAg₂₄(SR)₁₈] in which the central silver atom is replaced with platinum (Pt) were used as the sensitizers (Figure 3). These were combined with blue-emitting perylene or TIPS-anthracene as the emitters (Figure 3) to successfully observe and evaluate photon upconversion in solutions and solids (Figure 4).

In TTA-UC, which uses organic dyes as sensitizers, heavy metal ions are introduced into the dye scaffold to promote intersystem crossing (ISC) and elicit efficient photon upconversion. Interestingly, however, only very weak upconversion fluorescence was observed with the Ag₂₅(SR)₁₈ clusters, which are composed of heavy Ag atoms. Therefore, the central metal atom of Ag₂₅(SR)₁₈ was replaced with a Pt atom, which is heavier than Ag. This led to strong upconversion fluorescence when irradiated with near-infrared light of wavelength 785 nm (Figure 4a).

This study demonstrated that the metal cluster sensitizer discovered by the research group is capable of converting near-infrared light into blue light when combined with a suitable emitter, even in solid states (Figure 4b). This could someday be applied to photon upconversion materials to improve the efficiency of solar cells and photocatalysts. Metal clusters come in a wide variety of sizes, compositions, and shapes in addition to Ag₂₅ and PtAg₂₄, and because it is possible to control their electronic state and photophysical properties and make them hydrophilic, even more efficient metal cluster sensitizers will be possible in the future. This could have applications not only in the energy field, but also to medicine such as in bioimaging and optogenetics.

This study was carried out with the following support:

• JSPS grant-in-aid for scientific research (C) 20K05653
• JSPS grant-in-aid for young scientists (C) 20K15110
• Sumitomo Foundation grant for basic science research projects 170899